I used to not use salt in stock making, which is how I learned to do it in cooking school. But then I read somewhere that a bit of salt at the beginning of, say, a white chicken stock, help to coagulate the proteins and schmutz, which then are very easy to skim off the top.
Now I salt all stocks at the start; very judiciously. And I skim pretty often (or when necessary) during the first hour, before mirepoix is added.